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Section 7: Polymers—Materials Adapted from Nature
Polymers are a deceptively simple concept—large molecules made up of many small, uniform molecules linked together in long chains. Over the past century, polymer chemistry has produced an enormous number of products, including synthetic fabrics, spray-on insulation, waterproof films and coatings, carpet fibers, and all kinds of plastic goods. To understand why polymer chemistry is such a diverse and important field, it is helpful to know about polymers' basic properties and to see how many useful polymers occur in nature.
Polymers are molecular solids, held together by covalent bonds. They can have amorphous or crystalline structures. Polymers consist of long chains of molecules, often containing hundreds of thousands of atoms. Figure 13-11 shows the structure of an ethylene molecule and a polyethylene chain made up of ethylene monomers.
Figure 13-11. Polyethylene Chain
Polyethylene is produced by breaking the double bonds in ethylene molecules, freeing two electrons to form new single bonds with other ethylene molecules. Note that only a fragment of the polyethylene chain is shown; it continues in both directions.
© Science Media Group.
This structure makes many polymers very flexible, although some types that have cross-links between individual strands are rigid. Adding materials with lower molecular weights to a polymer alters its properties. It can make the base material harder by creating cross-links between polymer chains, or can make it softer by interfering with intermolecular forces and preventing crystals from forming. Additives that make a material softer or more fluid are called "plasticizers," alluding to the word "plastic" as an adjective—something that can be shaped or formed.
Figure 13-12. Vulcanization of Rubber with Sulfur
Vulcanizing rubber by heating it with sulfur locks together soft rubber molecules, making them harder and more durable.
© Science Media Group.
Many important natural materials are polymers, including wool, silk, and cellulose (plant fiber). Humans were working with and manipulating polymers long before they understood their chemical structure. For example, natural rubber is made by tapping rubber trees to extract latex, a milky fluid polymer, and letting it dry. Explorers brought it back from the Americas to Europe in the 18th century. English chemist Joseph Priestley, whom we met in Unit 1, found that this so-called "Indian gum" could erase lead pencil marks, and dubbed it "rubber."
Natural rubber was soft, and early rubber goods melted on hot days until the 1830s, when American merchant Charles Goodyear (1800–1860) found that treating rubber with sulfur and then heating it made it weatherproof. English scientist Thomas Hancock (1786–1865) adapted Goodyear's idea and patented it as vulcanized rubber in 1844. Vulcanization creates cross-links between polymer chains in rubber, making it more durable and water-repellent. (Figure 13-12)
Figure 13-13. The Structure of Silk (Yellow Squares Are Beta-Sheet Crystals)
Silk's unusual structure, which makes it very lightweight and strong, shows that hydrogen bonds can produce strong materials if they are arranged in the right ways.
© Science Media Group, adapted from an image by M. Buehler from the paper: S. Keten, Z. Xu, B. Ihle, M.J. Buehler, "Nanoconfinement controls stiffness, strength, and mechanical toughness of beta-sheet crystals in silk," Nature Materials, Vol. 9. pp. 359–367, 2010.
Silk is another natural polymer that is widely used worldwide, mainly for textiles. Silk threads are extremely light, but also very strong relative to their weight, as anyone knows who has seen beetles snared in a spider's web. It is also extremely ductile, so it can bend and stretch without breaking. Silk is made up of proteins, including some that form thin, flat structures called "beta sheets." Beta sheets can stack together in a crystalline formation, joined by hydrogen bonds. Hydrogen bonds are relatively weak; but when the bonds in silk fail, they quickly re-form. And silk beta sheets are stacked in arrangements that allow hydrogen bonds to reinforce adjacent chains, making silk very flexible and strong. (Figure 13-13)